Method of controlling long term safety of underground entry system by regulating formation of stress envelopes

ABSTRACT

The long term safety of underground rooms excavated in weak ground media which are subjected to large overburden and lateral earth pressures is regulated by controlling the formation of stress envelopes embracing an entry system. Within the entry system the rooms to be protected are placed parallel to the relief openings which are specifically designed to fail. An entry system designed with a predetermined safety period up to infinity is used for making extensive underground excavations safe and economical by supporting the overburden with an orderly development of stress envelopes. In order to create an entry system with a protected room or rooms, two relief openings are first excavated for the purpose of forming primary stress envelopes around the individual relief openings, then one or more protected rooms are excavated between the relief openings in such a manner that the primary envelopes are quickly transformed into a single secondary stress envelope which embraces the entire group of relief openings and protected rooms. A stress relieved zone is thereby created inside the secondary envelope which acts as a stable lining, forming a tubular shield against rock deterioration at the inside boundary of the stress envelope. The envelope supports the overburden and lateral earth pressures. Besides the protection of underground rooms, the stress envelopes may be utilized to control deformation and subsidence of the ground over a mined-out area by creating large protective stress envelopes in the main body of the overburden formation in the same manner as for the protection of underground rooms.

United States Eatent Serata 1 July 4, 1972 [54] METHOD OF CONTROLLING LONG TERM SAFETY OF UNDERGROUND ENTRY SYSTEM BY REGULATING FORMATION OF STRESS ENVELOPES Shosei Serata, 14 Calvin Court, Orinda, Calif. 94563 [22] Filed: Nov. 25, 1970 [21] Appl.No.: 92,768

[72] Inventor:

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 886,911, Dec. 22,

Primary Examiner-Dennis L. Taylor Attorney-Harris Zimmerman STRESSIED ZONE [57] ABSTRACT The long term safety of underground rooms excavated in weak ground media which are subjected to large overburden and lateral earth pressures is regulated by controlling the formation of stress envelopes embracing an entry system. Within the entry system the rooms to be protected are placed parallel to the relief openings which are specifically designed to fail, An entry system designed with a predetermined safety period up to infinity is used for making extensive underground excavations safe and economical by supporting the overburden with an orderly development of stress envelopes. In order to create an entry system with a protected room or rooms, two relief openings are first excavated for the purpose of forming primary stress envelopes around the individual relief openings, then one or more protected rooms are excavated between the relief openings in such a manner that the primary envelopes are quickly transformed into a single secondary stress envelope which embraces the entire group of relief openings and protected rooms. A stress relieved zone is thereby created inside the secondary envelope which acts as a stable lining, forming a tubular shield against rock deterioration at the inside boundary of the stress envelope. The envelope supports the overburden and lateral earth pressures. Besides the protection of underground rooms, the stress envelopes may be utilized to control deformation and subsidence of the ground over a minedout area by creating large protective stress envelopes in the main body of the overburden formation in the same manner as for the protection of underground rooms.

8 Claims, 8 Drawing Figures PATENTEDJUF4 I97:

sum 2 0f 2 INVENTOR. hosel emra ATTORNEY METHOD OF CONTROLLING LONG TERM SAFETY OF UNDERGROUND ENTRY SYSTEM BY REGULATING FORMATION OF STRESS ENVELOPES This application is a continuation-in-part of my application, Ser. No. 886,91 l, filed Dec. 22, 1969 and now abandoned, entitled Method of Controlling Safety of Underground Entry System by Regulating Formation of Stress Envelopes.

BACKGROUND OF THE INVENTION In various applications it is desirable to provide underground excavations which may be maintained safe without requirement of extensive artificial support structure. For example, in the mining of underground ore deposits which are surrounded by weak ground, conventional mining methods impose severe limitations on the economy and safety of ore recovery. In this regard, small room widths (often less than two to three times room height) and large ground pillar sizes (often more than five to times room height) have been adopted to reduce failure of the roof strata in the interest of safety. In general, extensive roof support structure is required whenever the reduction of room widths and enlargement of pillar sizes are not sufficient to secure the long tenn safety of the mine openings. The foregoing safety measures, of course, detract from the economy of ore recovery, not only from the standpoint of the cost of the support structure, and maintenance thereof, but in addition the reduced extraction of ore reserve and the limited room width which is not conducive to the efficient use of rapid mechanical mining methods.

As an alternative to the reinforced room and pillar method discussed above, a mining method has been devised which is variously termed retreat mining, total extraction mining, or "long wall mining". Basically, in accordance with this latter method, the ore body is extracted while the roof is allowed to cavein .uniformly and gradually, leaving small pillars behind for regulation of the cave-in speed in the process of retreating from the furthest perimeter of a given mining area toward the shaft area. The cave-in speed is suitably maintained so that the mining men and machines are at all times operating in a safe room in front of the advancing roof fall. Although the retreat mining method has the advantage of achieving relatively large room widths and small pillar sizes, and consequently a high extraction rate, the method is disadvantageous from the standpoint of failure to provide for controlling long term support of the over'burden. Consequently, the method is not acceptable to developing mines in which the controlled long term support of the overburden is required. For example, themethod can not be practiced in ground lying under or immediately above water bearing formations since a possible mine flood may result if the overburden or mine floor is significantly disturbed.

The retreat mining method was modified recently by blocking the extraction within a restricted area surrounded by large barrier pillars often more than 10 times the room height. This blocked retreat mining provides support of the overburden but fails to control the long term safety in the mined-out openings as all the openings are expected to cave-in. Therefore, this modified retreat mining still has the disadvantages of lacking full control of the overburden and security of the working area. Particularly in any developing mine, advance mining is preferred over retreat mining for many other reasons.

It will therefore be appreciated that in the present state of the art there is no method of providing safe underground rooms in developing mines having weak roof strata except to maintain the room width and extraction rate at the minimum acceptable level, while providing some auxiliary roof support structure. The economy of ore production is thereby limited. Furthermore, the excavation of large width rooms or tunnels, for whatever purpose, without extensive prohibitively costly support structure has been precluded with existing excavation methods.

SUMMARY OF THE INVENTION The general object of the present invention is to provide for an underground entry system with protected rooms of any desired period of structural safety, which can be used for the development of safe underground openings of excavations even in weak underground formations subjected to a high earth pressure, without requirement of artificial support structure. It is also to provide a method of controlling deformation and subsidence of the ground over a mined-out area for the safety of the mine.

In the accomplishment of the foregoing the intensity of the stress field in an underground formation in which protected rooms are to be excavated is reduced to an acceptable level by the controlled formation of stress envelopes. In this regard, a set of two relief openings are preliminarily excavated in the stressed ground, and such openings are designed to permit the roofs and floors thereof to fail at a preset time after their excavation for the purpose of forming a primary stress envelope around the individual relief openings. Protected rooms are then excavated parallel to the relief openings in the space between the two openings. The partitioning pillars between adjacent openings are designed to yield readily so as to quickly transform the two primary stress envelopes into a single secondary stress envelope which embraces the entry system, which consists of the entire group of two relief openings and one protected room and stably supports the overburden and lateral earth pressure for a predetermined period of time up to infinity. The stress in the entire'media inside the secondary envelope is relieved whereby the stress relieved mass acts as a stable lining structure forming a tubular shield against rock deterioration along the inside boundary of the secondary stress envelope.

An individual relief opening may also be formed by a multiplicity of sub-openings separated by one or more thin partition walls within the opening. Regardless of the presence of the partition walls, a single primary stress envelope is formed around the relief opening provided that the excavation of the sub-opening proceeds from one side to the other. The thickness of the partition walls is made sufficiently small so that the walls provide only a temporary safe working space protected from the quick fall of the failing roof slabs without altering the stress condition around the relief openings. An individual protected room may also be formed by a multiplicity of sub-rooms which are separated by thin separation walls. Contrary to the relief opening, the partition walls of the protected room are made for the convenience of air ventilation rather than delaying the roof slab fall.

The safety periods in the protected rooms of an entry system may be expressed by a time criterion depending upon the nature of the structural deterioration which may be expected in the protected rooms; namely, the time required for brittle fracture and plastic closure of the rooms beyond their intended use. Thus, the safety period is determined specific to the rock mechanics condition of the underground and intended usage of the protected rooms. For instance, an entry system to be used for the main traffic of a mine can be designed for a safety period not less than the expected life time of the mine.

The safety period of a protected room in a given entry system can be predetermined at any desired time by regulating the five basic parameters of entry system design; namely, width of relief openings, width of protected room, width of yield pillars, delay time of excavating the protected rooms and multiplicity of the sub-openings and sub-rooms. Further control of the safety period is made by introducing a geometric factor of relief opening intensity; that is, the ratio of a total width of relief openings to a total width of protected room or rooms in a given entry system. The relief opening intensity may be regulated in a range depending upon the rock mechanics condition of the ground. The larger the value of the factor, the greater the safety period for a given set of the five basic design parameters. By a systematic regulation of the above mentioned five basic parameters and one geometric factor, an entry system with a given protected room or rooms can be designed for any desired safety period ranging from less than a month to an infinite period of time under most rock mechanics conditions except where the rock media lying immediately above the protected rooms lack sufficient self-cohesion to remain intact, such as loose sand, flowing clay and very thin layers of separated rock media.

The process of transforming a set of two primary stress envelopes into a stable secondary stress envelope can be repeated by utilizing a set of two secondary envelopes to form one stable tertiary stress envelope which embraces all the openings and rooms involved. This compounding process of the stress envelopes can be further repeated many times to create a large stable stress envelope surrounding a part or the whole of a mined-out area of the underground. The shape and height of the final stable stress envelope can be regulated by the invented method so that the stress envelope is placed in the overburden at any desired formation which is suitable for maintaining the stress envelope stable. More than one of the extended stress envelopes can be linked in parallel at abutment pillars therebetween to provide a sheath of stable envelopes for controlling deformation and subsidence of the overburden formations, for the safety of the underground as well as that of surface structures. Such control permits safe excavation of a maximum areal space of the underground without disturbing the overburden formation beyond any predetermined level. This is particularly important wherever there is a source of ground water overlying the excavation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of a conventional underground excavation and attendant stress envelope with the lateral stress pattern along a vertical center line of the excavation superimposed thereof.

FIG. 2 is a representation similar to FIG. I depicting initial stages of preferred embodiment of the underground excavation method of the present invention.

FIGS. 3 and 4 are schematic representations similar to FIG. 2, but depicting subsequent stages of the method.

FIG. 5 is a schematic representation of the final stage of the method. 7

FIG. 6 is a view similar to FIG. 2, but illustrating a modified form of the invention wherein a plurality of sub-openings are utilized to provide the relief openings.

FIG. 7 is a view similar to FIG. 5, further illustrating a modified form of the invention wherein a plurality of subrooms are utilized to provide the protected room.

FIG. 8 is another view similar to FIG. 5 illustrating the compounding process of creating stress envelopes.

DETAILED DESCRIPTION OF THE INVENTION As shown in FIG. 1, when a horizontally extending underground opening 11 is excavated or otherwise formed, as for example, in underground formations, a stress envelope 12 is established substantially uniformly about the transverse cross section of the opening. The stress envelope represents the zone of excess tangential stress on over the initial stress (stressed zone) regarding the boundary of the openings. The lateral stress '1, which is the tangential stress 0's in the roof medium obtained along the centerline of the individual openings concerned is generally distributed as indicated by the curve 13. In this regard, it is to be noted that the stress is highly concentrated in the vicinity of the envelope immediately adjacent to the opening 11. Of more importance, the stress gradient is extremely sharp or steep immediately adjacent to the opening. It has been found that such a concentration and large gradient of lateral stress, not gravitational forces, are directly responsible for the mechanism of roof failure with such a conventionally formed opening 11. To minimize the possibility of roof failure, it has been the usual practice to limit the width of the opening and/or erect costly support structure, both of which measures are undesirable from an economic standpoint, and in some instances, even inadequate, for the reasons discussed hereinbefore. The present invention obviates the foregoing difficulties by providing design for an entry system in which relief openings and protected rooms are excavated close together so that a protective stress envelope embracing the entry system is controllably formed serving to secure the longterm room safety for any desired period against collapse or failure of the surrounding media without requirement of auxiliary support structure.

Considering now the underground excavation method of the present invention in some detail with respect to the illustrated forms thereof in the drawings, the method will be seen to generally comprise preliminarily excavating at least two relief openings in an underground medium in which one or more prospective protected rooms are to be excavated and regulating the degree of roof and floor failure of each relief opening to controllably develop a primary stress envelope about each relief opening, and excavating one or more of the protected 1 rooms in the medium between two previously formed relief openings to define yield pillars therebetween of a size adapted to yield readily and thereby quickly transform the individual primary stress envelopes associated with the relief openings and protected rooms into a single secondary stress envelope encompassing the entry system consisting of the entire group of relief openings and protected rooms, such secondary envelope having a stress relieved zone within the entire media encompassing the same. The secondary envelope serves to stably support the overburden and lateral earth pressures about the protected rooms without requirement of auxiliary support structure, while the stress relieved zone acts as a stable lining structure which forms a shield against rock scaling along the inside boundary of the secondary stress envelope.

The method outlined above will bebetter understood by consideration of the preferred embodiment thereof illustrated in FIGS. 2-5. More particularly, in accordance with this embodiment of the method, two relief openings 14 and 16 are excavated or otherwise formed to extend horizontally through the underground media. The relief openings are parallel and laterally spaced on opposite sides of a horizontally extending region 17 in which it is proposed to provide a protected room with a predetermined safety period. By virtue of the relief openings, primary stress envelopes 18 and 19 are established therearound. Initially, the primary stress envelopes l8 and 19 are relatively flat and the tangential stress 21 is concentrated around each opening in a relatively narrow stressed zone, as indicated by the peak in the lateral stress distribution curve 21 shown in FIG. 2. However, the roof and floor 22 and 23 of each relief opening are made wide enough to fail at some predetermined time in a regulated manner to thereby relax the media overlying and underlying the opening. After a time period the relaxation of the roof and floor media has progressed to the extent depicted in FIG. 3. Each primary stress envelope bulges outwardly, resulting in a reduction of the peak intensity of the tangential stress and at the same time an increase of the thickness of the stress envelope as illustrated by the lateral stress distribution curve 21. The excess tangential stress is consequently distributed over a relatively wider and deeper stressed zone adjacent to the relief opening. Relaxation may be permitted to continue until the roof media over each relief opening collapses and falls on the floor of the opening, as.indicated at 24 in FIG. 4, thereby leaving a newly formed stable natural roof above. This results in a further flattening of the peak in the lateral stress distribution curve and greater widening of the stressed zone.

It is of importance to note from the foregoing that the development of the primary stress envelopes about the relief openings is controlled by regulation of the degree of room and floor failure thereof. More particularly, regulation is obtained by predetermination of the width of a relief opening with respect to its height, width of the prospective protected room and the rock mechanics condition of the surrounding ground.

The greater the width of the relief opening, the faster and deeper the stress relaxation of the roof and floor media, and consequently the taller the height of the primary stress envelope developed after a given period of time. Thus, the degree of roof and floor failures may be regulated at any desired level ranging from a slight stress reduction in the media to massive roof and floor failures, to in turn control the development of the primary stress envelope to whatever extent is desired.

Subsequent to the development of the primary stress envelopes l8 and 19 in the manner just described, the proposed protected room 26 is excavated or otherwise formed in region 17 laterally intermediate and parallel to the relief openings 14 and 16, as shown in FIG. 5. The partitioning pillars 27 defined between the protected room and relief openings are designed to yield readily without failure against further deformation of the surrounding ground media. More particularly, the yield pillars 27 are made narrow, depending upon the yielding capacity of the pillar media relative to the earth pressure. Yielding of the pillars simultaneously transforms all of the primary stress envelopes, i.e. the envelopes 18 and 19 associated with relief openings 14 and 16 and a primary stress envelope (not shown) associated with the protected room 26, into a single secondary stress envelope 28 encompassing the entry system which consists of the entire group of relief openings and protected rooms, and relieves the stress in the entire media inside the secondary stress envelope, thus establishing a tubular stress relieved zone 29 therein. It is to be noted that the secondary envelope 28 is of generally oval or laterally elongated elliptical configuration extending arcuately inward from the relief openings in overlying relation to the protected room. The associated distribution of lateral stress '1. along the vertical centerline of the protected room 26 is as depicted by the curve 32. The curve includes a peaked portion, vertically outward from the protected room representative of the stress arch 28, merging inwardly with a relatively flat, gently curving minimum portion representative of the stress relieved zone 29 of the envelope. The stress gradient in the relieved zone will be observed from curve 32 to be relatively flat and thus small compared to that of the stress distribution curve 13 of FIG. 1. Consequently, the opening 26 is protected against overburden and lateral earth pressures and is rendered almost permanently stable by the high concentration of tangential stress in the secondary stress envelope which surrounds the protected room in outwardly spaced relation thereto. By virtue of the tubular stress relieved zone 29 there is very little stress concentration in the media immediately bounding the protected room, whereby the stress relieved mass functions as a cylindrical shield against rock deterioration which would normally be expected on the inside boundary of a stress envelope. Moreover, the yield pillars 27 serve to prevent a buildup of stress in the entire roof and floor mass inside the stress envelope by creep deformation of the yielded pillars; at the same time the residual strength of the yielded pillars provides adequate support to the stress relieved media immediately over the protected room.

The safety period of the protected room 26 in the entry system of FIG. is controlled by regulating the five basic design parameters of the entry system; namely, widths of yield openings 14,16; width of the protected room 26; widths of yield pillars 27; timing of excavation of the protected room and multiplicity of sub-openings and sub-rooms with respect to the rock mechanics condition of the surrounding ground. In this example, as shown in FIGS. 2,3,4 and 5, both the openings and room are made from a single excavation. However, the individual relief openings and protected rooms may be made of sub-openings and sub-rooms as illustrated in FIGS. 6 and 7, presently to be described. The opening widths determine the extent of the primary stress envelopes (FIG. 6) and the delay time of the room excavation determines the intensity of the relaxation of the media inside the primary stress envelopes. The width of the protected room determines the shape and size of the secondary stress envelope, the widths of the yield pillars determine the speed of transforming the primary stress envelopes into a single secondary stress envelope 28. The multiplicity of the openings and rooms enables the widths of the openings and rooms to be greater than what is possible in a single excavation. The rock mechanics condition of the underground is characterized by the three basic rock mechanics factors of initial underground stress field, rock properties and geometry of intended excavation of the entry system.

It will be appreciated that the concentration of stress in the secondary stress envelope 28 and thus the strength thereof, as well as the height of the stress relieved zone 29 are determined by the extent to which the controlled development of the primary stress envelopes 18 and 19 has progressed prior to the formation of the protected room 26. Thus, with the controlled formation of the secondary envelope and relieved zone by the previously described regulation of the five design parameters the room 26 is protected for any desired safety period even when formed in weak ground. An entry system with the protected room formed in the manner hereinbefore described is suited to use as the main entry system of a mine, or other underground excavation which is expected to stay safe for underground traffic and free from roof maintenance work for the entire life thereof.

In FIG. 6, a modified arrangement for providing the relief openings is illustrated, such openings generally corresponding to relief openings 14 and 16 of FIG. 2. In this arrangement, the relief openings, generally designated at 84 and 86 are excavated or otherwise formed on opposite sides of a region 87 in which the protected room is to be provided. Each of such relief openings is formed from a plurality of sub-openings, e.g. sub-openings 88 and 89 constituting opening 84, and subopenings 90, 91 and 92 constituting opening 86. Separating the sub-openings are thin partition walls 93. Irrespective of the number of sub-openings, single primary stress envelopes 94 and 95 are established around the respective relief openings 84 and 86.

The foregoing multiple sub-opening relief openings may be used with the single protected room illustrated in FIG. 5, or, as shown in FIG. 7, the protected room 96 may be formed by a plurality of sub-rooms, e.g. 97,98, separated by their partition walls 99. Here again, the two primary stress envelopes associated with the two relief openings, fon'n a single secondary stress envelope 100 encompassing the entire group of relief openings and protected rooms.

As hereinbefore stated, the process of transferring a set of two primary stress envelopes, e.g. envelopes 18 and 19, into a stable secondary envelope, e.g. envelope 28, can be repeated by utilizing a set of two such secondary envelopes to form a stable tertiary envelope which embraces all of the openings and rooms involved. This compounding effect is illustrated in FIG. 8 of the drawings. As shown, a protected room 26 is shown between a pair of spaced secondary envelopes 28' and 28". These secondary envelopes form a stable tertiary envelope 110. Such compounding can be repeated by using pairs of the last created stress envelopes to form a fourth stable stress envelope 112, ad infinitum.

Although the invention has been described hereinbefore primarily with respect to excavation in a horizontal plane formation, it should be noted that the principal and technique thereof are equally applicable to any inclined formation or formations, regardless of the angle of inclination, as well as to three-dimensional space in the underground. Thus, it is not intended to limit the invention except by the terms of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of controlling long term safety on an underground entry system comprising the steps of preliminarily forming two relief openings in an underground medium between which a prospective protected room is to be formed to develop a primary stress envelope about each relief opening; and subsequently forming said protected room in said medium between said two relief openings to define yield pillars therebetween, each relief opening having a primary stress envelope therearound and said pillars being of a size to yield readily without failure to thereby quickly transform said primary stress envelopes about said relief openings and protected room into a single secondary stress envelope encompassing the entire group of said relief openings and protected room with a stress relieved zone within the entire media encompassed by said secondary stress envelope, whereby the secondary stress envelope stably supports the overburden and lateral earth pressure about the protected room and the stress relieved zone forms a tubular shield against rock deterioration along the inside boundary of the secondary stress envelope.

2. A method according to claim 1, further defined by regulating the degree of roof and floor failure of each relief opening with respect to time delay in excavation of the protected room to controllably develop the primary stress envelope thereabout.

3. A method according to claim 2, wherein regulation of the degree of roof and floor failure includes predetermining the width of each relief opening relative to its height, width of proposed protected room and the rock mechanics condition of said underground medium.

4. A method according to claim 1, wherein each of said yield pillars is defined to have a width so that the pillars yield readily to transform all of the primary stress envelopes developed around the individual openings and room into a single secondary stress envelope embracing the entire system and at the same time provide an adequate support to the weight of the relaxed media immediately above the protected rooms.

5. A method as set forth in claim 1 in which at least one of said relief openings is formed of a plurality of generally parallel sub-openings with thin partition walls between such subopenings.

6. A method as set forth in claim 5 in which both of said relief openings are formed of said sub-openings.

7. A method as set forth in claim 1 in which said protected room is formed of a plurality of sub-rooms separated by thin partition walls. 3

8. A method as set forth in claim 1 in which a second protected room is formed between a pair of previously formed secondary stress envelopes to transfonn said secondary stress envelopes and second protected room into a tertiary stress envelope encompassing the entire group of secondary stress envelopes and second protected room. 

1. A method of controlling long term safety on an underground entry system comprising the steps of preliminarily forming two relief openings in an underground medium between which a prospective protected room is to be formed to develop a primary stress envelope about each relief opening; and subsequently forming said protected room in said medium between said two relief openings to define yield pillars therebetween, each relief opening having a primary stress envelope therearound and said pillars being of a size to yield readily without failure to thereby quickly transform said primary stress envelopes about said relief openings and protected room into a single secondary stress envelope encompassing the entire group of said relief openings and protected room with a stress relieved zone within the entire media encompassed by said secondary stress envelope, whereby the secondary stress envelope stably supports the overburden and lateral earth pressure about the protected room and the stress relieved zone forms a tubular shield agaiNst rock deterioration along the inside boundary of the secondary stress envelope.
 2. A method according to claim 1, further defined by regulating the degree of roof and floor failure of each relief opening with respect to time delay in excavation of the protected room to controllably develop the primary stress envelope thereabout.
 3. A method according to claim 2, wherein regulation of the degree of roof and floor failure includes predetermining the width of each relief opening relative to its height, width of proposed protected room and the rock mechanics condition of said underground medium.
 4. A method according to claim 1, wherein each of said yield pillars is defined to have a width so that the pillars yield readily to transform all of the primary stress envelopes developed around the individual openings and room into a single secondary stress envelope embracing the entire system and at the same time provide an adequate support to the weight of the relaxed media immediately above the protected rooms.
 5. A method as set forth in claim 1 in which at least one of said relief openings is formed of a plurality of generally parallel sub-openings with thin partition walls between such sub-openings.
 6. A method as set forth in claim 5 in which both of said relief openings are formed of said sub-openings.
 7. A method as set forth in claim 1 in which said protected room is formed of a plurality of sub-rooms separated by thin partition walls.
 8. A method as set forth in claim 1 in which a second protected room is formed between a pair of previously formed secondary stress envelopes to transform said secondary stress envelopes and second protected room into a tertiary stress envelope encompassing the entire group of secondary stress envelopes and second protected room. 